In underground mining operations, excavated rock wall and ceiling support is commonly employed so as to prevent or reduce the occurrence of rock collapse in excavated areas, such as tunnels, drifts or mine shafts. Rock bolts placed into the rock, generally using mechanical anchors and/or grouts, and positioned at intervals along the excavation may offer a primary form of protection against unplanned rock falls or bursts. Secondary rock wall and ceiling support against smaller rock falls is commonly provided using a combination of a metal wire mesh installed against excavated rock faces with rock bolts and a hardened cementitious material, which is commonly a sprayed concrete such as shotcrete or gunite, to bond to and cover the wire mesh. However, development of thin spray-on liners (TSL's) as a secondary ground support material has begun in recent years. Such TSL's may be formed using a high performance polyurea coating containing a reactive polyurethane or other suitable polymer dispersed into a polymerizable (i.e., capable of undergoing polymerization) diluent.
As ground support materials, combination mesh and shotcrete can exhibit one or more disadvantages or shortcomings. For example, the application of shotcrete onto mesh can be cumbersome and fairly labor intensive, especially in deep mining applications where it can become increasingly more difficult to navigate the large trucks, materials and machinery used for this purpose. Linings produced by combination mesh and shotcrete can also tend to be brittle and lacking in tensile (as opposed to compressive) strength and toughness. Such tensile weakness may render shotcrete-based linings more prone to fracture during mine blasting or other underground operations that cause significant flexing of the underlying rock. This effect may be exacerbated if the wire mesh is not installed flush with an excavated rock face. Additionally, shotcrete may have long dry times to reach full tensile strength of about 1 MPa, which can adversely affect productivity by extending delay times between successive rock blasts while the shotcrete is hardening.
Compared to cementitious ground support materials, such as shotcrete or gunite, TSL's may offer a number of advantages. For example, spray-on liners may offer superior tensile strength (e.g., up to or above 2.5 MPa) with significantly shorter cure times (e.g., as little as 20 seconds) and with thinner resulting material layers. Application of TSL materials to excavated rock surfaces may also be greatly simplified due to reduced material bulk, which may be up to an order of magnitude less volume than shotcrete. Elimination of wire meshing that is commonly used in conjunction with shotcrete or gunite may also confer benefits in its own right, for example, because corrosion of wire meshing is no longer of concern. Handling large sheets of wire mesh is eliminated in confined underground spaces. Further benefits of TSL materials include that its finished surface is usually smoother than shotcrete and therefore less likely to hold mine dust, which may lead to a cleaner and safer working environment. Commonly TSL materials are also manufactured to have a bright colour making the liner highly visible and contributing to a brighter mine environment that can reduce lighting requirements and improve safety conditions.